Pressure swing adsorption (PSA) processes are well-known for separating gas mixtures by selective adsorption of one or more components of the gas mixture on an adsorbent. By way of example, U.S. Pat. No. 3,430,418 discloses a PSA process employing four adsorbent columns arranged in parallel flow relationship, with each column proceeding sequentially through a multi-step cycle, consisting of adsorption, two concurrent depressurizations, countercurrent depressurization, countercurrent purge and two countercurrent pressurization steps. The purpose of such a complex process design is to improve the separation efficiency. The operation of the process requires at least four adsorbent columns interconnected by several gas headers and many switch valves in order to have a continuous feed stream and product stream. It is apparent that the total cycle time required for completion of the steps of this process, which include flowing large quantities of gas in and out of the adsorbent columns and transfer of gas between columns, will be significant. For example, one embodiment of U.S. Pat. No. 3,430,418 for production of oxygen from air by selective adsorption of nitrogen on a zeolite requires 240 seconds to complete one cycle.
Generally, PSA systems require multi-step cycles and multicolumn design using several gas headers and switch valves in order to obtain high degree of separation and continuity of feed and product gases. Consequently, they require total cycle times of one to several minutes. Typically, at least one adsorbent column in the system undergoes the adsorption step at all times during the cycle so that the time slot for the adsorption step is given by the ratio of the total cycle time for the multi-step process to the number of adsorbers used in the process. It follows that a typical adsorption time for the PSA process is in the order of minutes and the adsorber size needs to be large enough to handle the feed gas for that duration. Furthermore, the adsorbent particle sizes used in commercial adsorbers are typically between 1.5-3 mm diameter, so that the pressure drop in the adsorber is small. This can increase the diffusional mass transfer resistance for adsorbing components of feed gas mixture into the adsorbent particle and create a mass transfer zone (MTZ) of significant size. Since the separation efficiency in the MTZ is much reduced, the adsorbers are made of certain size so that the ratio of MTZ length to the adsorber length is small. Typically, the commercial adsorbers are 5-30 feet in length. The net result is a markedly increased and costly adsorbent inventory in situ for a given separation duty. Approaches that can reduce capital cost (adsorber vessels, piping, switch valves and related plumbing) and that can reduce adsorbent inventories are quite in order.
One method to achieve this goal is to use a rapid pressure swing adsorption (RPSA) process as described by U.S. Pat. No. 4,194,892, operating with a single adsorber and much reduced gas pipe lines and switch valves and a very fast cycle time of seconds. This process uses a three step cycle consisting of introducing the compressed feed gas into the adsorber at the feed end, for a very short period of time (seconds or fraction of seconds), then suspending feed introduction for a period of less than 10 times the feed introduction period and then countercurrently depressurizing the column to near ambient pressure in a time period which is at least twice as long as the feed introduction period. The feed introduction is suspended during the last two steps and it is reintroduced after the depressurization step in order to start a new cycle. A continuous product gas enriched in the less strongly adsorbing component of the feed gas mixture is withdrawn through the product end of the column during the entire cycle.
Although this process reduces adsorbent inventory by using a very short cycle time, it has several key deficiencies:
(a) The feed introduction is discontinuous which is an impediment for a commercial process, in particular, if a compressor is used to compress the feed gas;
(b) The desorption step (depressurization) is discontinuous. Thus, if the desorbed gases constitutes the main product then its flow is disruptive;
(c) The desorption of the absorbed components of the feed mixture is caused by pressure reduction and by back purging (countercurrent) by flow of a portion of the less strongly adsorbed component of the feed mixture which is separated and collected in the column towards the product end during the first two steps of the process. In order to supply a sufficient quantity of back purge gas, only a very small section of the column is used to hold the more strongly adsorbed component of the feed gas and the remaining column is used to hold the less strongly adsorbed component, thus, not utilizing the entire separator capacity of the column;
(d) The design of the column requires a critical relationship between its length, individual cycle times for the steps, pressure ratio between feed and desorption steps which can very much complicate the operation of the process due to fluctuations in feed gas composition, pressure and, or temperature which are common in industrial practice. This critical relationship between the operating variables is established by the requirement described in (c) above.
Another example of a RPSA process is described in U.S. Pat. No. 4,194,891 which is designed to produce an oxygen-enriched product gas from air by adsorption of nitrogen on a 5 Angstrom zeolite. It employs two or three adsorption columns arranged in parallel flow connection which undergo the sequential steps of adsorption, feed suspension, countercurrent desorption and purging, and one or two countercurrent pressurization steps with oxygen enriched product gas. A feed introduction time of 0.1 to 6.0 seconds is used. This process, therefore, appears to use the conventional design of a multicolumn, multi-step PSA system, except that the cycling is done fairly rapidly. Significant scale-up of this process may be questionable due to the reasons discussed earlier.
The present invention discloses a novel RPSA cycle and hardware arrangement which can overcome some of the shortcomings described above.
The new RPSA process provides continuous introduction of feed gas into the adsorber, continuous withdrawal of a stream enriched in the less strongly adsorbed component of the feed mixture, continuous withdrawal of a stream enriched in the more strongly adsorbed component of the feed mixture, very efficient use of the adsorbent capacity and yet significantly reduces the adsorbent inventory and the requirements for gas lines, adsorber vessels and switch valves.
These and other aspects and features of the invention will become apparent to one skilled in the art from the specification, claims and drawing appended hereto.